1. Field of the Invention
The present invention relates to a semiconductor device, and in particular, to a voltage controlled generator for a semiconductor device which makes it possible to actively control the pumping period of a bias voltage supplied to a semiconductor device using a voltage controlled oscillator.
2. Description of the Conventional Art
Recently, the semiconductor circuit is designed to have a characteristic that a substrate bias voltage is changed from a ground voltage GND to a negative bias voltage for thereby supplying a VBB voltage for the reason that the VBB voltage decreases a substrate bias effect and a junction electrostatic capacity of a threshold voltage, increases a punch through voltage and prevents a data damage due to undershoot.
The conventional substrate bias voltage (VBB) generation circuit is formed of a detector detecting a substrate bias voltage, a ring oscillator formed of an odd number of inverters and a charge pump. In the conventional circuit, when a VBB level is detected by the VBB detector, the ring oscillator is operated at a predetermined oscillation period, so that the VBB voltage generated based on the charge pump is supplied to the semiconductor substrate.
The thusly constituted VBB generation circuit should stably generate a substrate bias voltage level and a large Icp current based on a low electric power.
However, in the conventional VBB generation circuit, since a ring oscillator is operated at a constant oscillation period, if a substrate bias voltage variation occurs, the VBB voltage level of the charge pump does not quickly reach the substrate bias voltage, so that the efficiency of the charge pump is decreased, and the power consumption is increased.
In addition, the conventional step-up voltage generation circuit has a characteristic that a high voltage pulse higher than an external power voltage Vcc is applied to a semiconductor device for thereby compensating the threshold voltage loss of the transistor. The above-described conventional step-up voltage generation circuit is formed of a detector detecting the level of a step-up voltage VPP, a ring oscillator, and a charge pump. The step-up voltage generation circuit detects the VPP voltage level by the detector like the substrate bias voltage generation circuit and opens/closes the ring oscillator for thereby determining the operation of the charge pump.
Therefore, when the step-up voltage generation circuit is formed of a ring oscillator as a frequency oscillation device, since the signal period of the oscillator is constant, it is impossible to implement a fast pump speed of the charge pump when the level of the step-up voltage is decreased below a reference level for thereby decreasing the efficiency of the charge pump and increasing the power consumption.